Telephone transmitter



July 21, 1925. 1,546,749

' .17 G. ROBERTS TELEPHONE TRANSMITTER Filed Nov. 5, 1921 II M 4 v u u /3 o /7 o on 8 II II 4 Patented July 21, 1925:.

UNITED" STATES PATENT OFFICE.

Joan a. mum, on norms runny, NEW

YORK, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF YORK, N; Y., A CORPORATION OF NEW YORK.

I TELEPHONE TRANSMITTER.

Application filed November 3, 1921. Serial No. 512,428.

To all whom it may concern} Be it known that I, JOHN G. Rename, a citizen of the United States, residing at Dobbs; Ferry, in the county of Westchester, State of New Yoi k;-have, invented certain new and useful Improvements in Telephone Transmitters,' of which the following is a full, clear, concise, and exact description.

This invention relates to telephone transmitters and its object is to provide a transmitter which shall be relativelyv unresponsive to objectionable noises in the vicinity thereof in comparison with its sensitiveness to sounds :which it is desired to transmit.

Transmitters belonging to this class are especially adapted for use in situations w ere excessive noise in the vicinity makes it diflicult to use a telephone. A notable example of such a condition is found on aeroplanes where the noise of the motor exhaust renders the ordinary transmitter unsatisfactory forurposes ofcommunication.

It has con suggested that the effect of this extraneous noise ma beartia'lly nullified by exposing both si es o the transmitter diaphragm to the sound waves. The

transmitter of this invention embodies this,

princi 1e in an improved construction where y the main diaphragm is protected equally on both sides from objectionable sound vibrations by a baflle which screens the diaphragm from noise frequencies, but

which is designed to permit the comparativel unobstructed passage therethrough of voca or similar sounds. .This baffle is furs thermore 'formed in such a way as to roduce with the diaphragm a chamber w 'ch is resonant at a frequency of about 1300 vibrations per second.

In the preferred form one of the baffles is used on eachside of the transmitter diaphragm and two similar microphone buttons are mounted on the two sides of the diaphragm respectively. There is thus produced a balanced structure which will be com aratively unresponsive to sounds striking 0th sides of the diaphragm simultaneously, but which will respond to high frealiiency sounds localiz'ed on one side of the when From the following detailed descri tion of the invention other objects and ad vantages will become apparent.

One form of the invention, constructed in accordance with the general principles and embodying the features set forth above, is shown in vertical cross-section in the accompanying drawing.

Referring to the drawing, 1 is a diaphragm preferably circular in outline and made of an elastic substance, such as thin metal, hard rubber, or like substanceswhich are ordinarily employed in the construction of diaphragms.

Supporting'the diaphragm are two face members 2 and 3, which are clamped together by means of screws 4. Two rin washers 5 and'6 of oiled linen are inserte between the diaphragm and the face members 2 and 3, respectively, to assist in maintaining the proper spacing therebetween to form a thin resonating chamber on each side of thediaphragm and to damp the vibrations at the edge of the diaphragm.

The face -members 2 and 3 are of substantial thickness and weight so as not to vibrate to any marked extent. A large number, of apertures? are provided through each face member to the resonating chambers. The diameter of these holes is preferably less than theirlength as this is the form which has been found by experiment to operate most successfully in preventing the admission of violent noisesof an objectionable character. 7

Integral with each face member 2 and. 3 is a projecting hub 8 having an annular flange 9 perpendicular thereto. Supported on each of the flanges 9'is a shouldered bushing 10 threaded at one end to receive a nut 11 for securing it in place. The bushings 10 are insulated from the face mem- 16 are separated from the fixed electrodes by felt rings 17 which serve to form chambers'for the variable resistance element 18 which is preferably granular carbon.

The movable electrodes 16 are held firmly against the diaphragm 1 by a proper adjustment of the shanks of the supports 13.

By the structure above described there is provided a thin resonant chamber between each side'of the diaphragm and theface members 2 and 3, communlcation to the chambers from the outer air being afforded by the apertures 7. In practice it has been found that violent sound vibrations or those having great amplitude are excluded from the diaphragm when the resonant chambers have a depth of .015 inches or less the diaphragm if they are within the voice range frequencies and not of a violent character such as the perforations of the facemembers exclude.

The reason for fixing the resonance of the chambers atapproxlmately 1300 vibrations per second is that it has been found that the power of the vowel sounds produced bythe human voice is at a maximum when their fundamental pitch has approximately this vibratory rate. At this rate also the carbon microphones respond at their highest efficiency. The uniformity of resonance in the vibratory system thus establishes a condition most favorable .to the transmission of vocal sounds.

The face members 2 and 3 perform the combined functions of a means for supporting the diaphragm, a bridge upon which the microphone elements are mounted, and a means cooperating with the diaphragm to provide damping therefor. It is desirable that the members 2 and 3 be rigid and light and preferably made of aluminum. When the diaphragm is constructed of conductive material, electrical connection to the movable electrodes 16 may be made by means of a suitable binding post (not shown) mounted on either one of the face members 2, 3. Conn'ection'with the rear electrodes may be made through the supporting members 13.

The transmitter is preferably associated with the well known puslrpull circuit arrangement. The connections for such a circuit are illustrated diagrammatically in the drawing. The rear electrodes 15 are each provided with individual conductors 19 and 20, and the front electrodes 16 have a common lead 21. The circuit for one -From the location of the source. 23 in the circuit it will be seen that the current flows in-opposite directions through thetwo halves of the primary winding. Therefore, the fiuctuations of current in the two halves of the primary winding produce additive inductive effects in the secondary winding of the coil 22.

What is claimed is:

1. In a telephone transmitter, a diaphragm, a chamber of similar sound characteristics on each side thereof, said chambers being resonant to substantially the same vibrations at voice frequencies and each provided with apertures thcrethrough to selectively admit such vibrations andexclude vibrations of other frequencies, and a pair of microphone elements arranged symmetrically with respect to the diaphragm and adapted to be actuated thereby.

2. In a telephone transmitter, a diaphragm, and a face member on each side of the-diaphragm and slightly separated there from, the face members being provided with apertures of greater length than width.

3. In a telephone transmitter, a diaphragm adapted to be actuated from either side, means forming a thin chamber therewith on each side thereof, each chamber being resonant to voice vibrations and provided with equal access to space external the transmitter.

4. In a telephone 1 transmitter, a diahragm, and equal resonating means for each side of the diaphragm resonant to voice frequencies.

5.- In a telephone transmitter, a diaphragm adapted to be actuated and equally responsive from either side, equal resonating means for each side of the diaphragm, and a similar microphone mounted on each side of the diaphragm to provide a symmetrical load on each diaphra m.

G. In a telephone transmitter, 21 diaphragm equally responsive from either side, a pair of microphones arranged symmetrically with respect to the diaphragm and adapted to be actuated thereby, and a single means for supporting the microphones. and the diaphragm and fonproviding a resonating chamber on each side of the diaphragm. said resonating chambers being of the same proportions.

'Z. In a telephone transmitter, a diaphragm, and means on each side of the diaphragm permitting substantially equal Current is supplied to both access thereto for sounds within a certain phone adapted to be actuated by the diarange of vibratory frequency and intensity phragm.

and for excludin other soun I In witness whereof, I hereunto subscribe 10 8. -In a telep one transmitter', a diamy name this 28th day of October A. D 5 phragm, equal resonating-meansmfor each 1921. side of the diaphragm resonant to frequencies within the woice range, and a micro- JOHN G. ROBERTS. 

